1. Field of the Invention
This invention relates to an aluminum alloy material having an excellent sea water corrosion resistance and a plate heat exchanger using the alloy material in or as a heat transfer unit making use of sea water as cooling water.
2. Description of the Related Art
Aluminum alloys have high specific strength and high heat conductivity, for which they have been widely used as a material for small-sized, lightweight heat exchangers. Typical of heat exchangers using an aluminum alloy material are fin and tube heat exchangers employed in domestic air conditioners, automotive radiators and the like. In contrast, industrial plate heat exchanger using sea water as cooling water make use generally of titanium and thus, the use of more economic aluminum alloys has been under investigation.
Such a plate heat exchanger having a heat transfer unit where sea water is provided as cooling water is exposed to a severe corrosion environment when used in a sea water environment. This is because highly corrosion-resistant titanium has now been in use. An aluminum alloy material has a high corrosion resistance on its own. Nevertheless, if an aluminum alloy is used in such a plate heat exchange in place of titanium, a further adequate anticorrosive measure has to be taken.
In general, for an anticorrosive measure of an aluminum alloy material for such a type of plate heat exchange, there is used, aside from the formation of an anodized oxide film, means such as of electrolytic protection, coating film formation using paints and the like. With the case of application to heat exchangers, means of adding inhibitors to cooling water is used.
However, plate heat exchangers are of a transient type, meaning that cooling water is discharged to outside of a system after passage through an apparatus and thus, cooling water is not used by circulation. Thus, an anticorrosive measure using an inhibitor is not proper, but an anticorrosive measure using paint film formation is economically appropriate.
On the other hand, various types of coating films on an aluminum alloy material for heat exchanger have been proposed including inorganic coating films, organic coating films, organic-inorganic hybrid coating films and have now been actually employed. Coating film formation means for heat exchangers have been proposed, for example, in Japanese Laid-open Patent Application Nos. 2003-88748 and 2004-42482 (which may be sometimes referred hereinafter to as Patent Literatures 1 and 2, respectively).
Patent Literature 1 refers to the formation of a polyaniline film wherein the film is not applied to plate heat exchangers using seat water, to which the present invention is directed, but is applied to an aluminum alloy material for fin and tube heat exchangers used in domestic air conditioners, automotive radiators and the like.
Patent Literature 2 discloses the formation of a coating film with improved adhesion wherein there is used, as a composite primer layer, a coating film that is obtained by subjecting, to boehmite treatment or silicate treatment, an aluminum alloy material for fin and tube heat exchangers, like Patent Literature 1, used in domestic air conditioners and automotive radiators. Moreover, in “Proceedings for Meeting for Zairyo-to-Kankyo, Self-repairing Property of Anticorrosive Coatings for Aluminium Alloys” written by Akihiro Yabuki, Hiroyoshi Yamagami, Takeshi Oowaki, Kiyomi Adachi and Koji Noishiki, p. 3-4 (2004) (which may be referred to hereinafter as Non-patent Literature 1), it is disclosed that when used as an anticorrosive coating film for a transient type of heat exchanger, a trifluoride resin exhibits self-repairability.
In Japanese Laid-open Patent Application No. 2006-169561 (which may be referred to hereinafter as Patent Literature 3), there has been proposed, as an improvement of trifluoride protective coating, a self-repairing aluminum alloy protective coating which is made of a trifluoride resin containing 0.1 to 10 vol % of at least one member selected from zinc, titanium, manganese, aluminum and niobium. This is a measure for the fact that with a heat exchanger using sea water as a cooling medium, the surface of the heat exchanger is liable to be damaged, with the tendency that when surface damage is once induced, the damage is abruptly enlarged by a vigorous corrosive action with sea water. More particularly, it is stated that the trifluoride resin anticorrosive coating film containing a powder of the above-mentioned metal has self-repairability when the coating suffers damage.
The coating film of Patent Literature 1 is sufficient for an improvement in corrosion resistance of fin and tube heat exchangers used for domestic air conditioners and automotive radiators. However, when used in plate heat exchangers using sea water, to which the invention is directed, such a coating film is unsatisfactory with respect to the corrosion resistance in a saline environment such as of sea water.
In contrast, the trifluoride resin anticorrosive coating films of Patent Literature 3 and the report made by Akihiro Yabuki et al have, in their own nature, a more excellent corrosion resistance to sea water than the coating film of Patent Literature 1 and other types of anticorrosive films such as an anodized film and other types of coating films. However, when applied to a plate heat exchanger using sea water, to which the invention is directed, there arises a problem in that they degrade in adhesion (adhesion durability) to aluminum alloy materials in long-term use and are not thus reliable.
The degradation problem of adhesion (adhesion durability) to aluminum alloy materials in long use likewise occurs in primer or underlying treatment that is directed to heat exchangers used in domestic air conditioners and automotive radiators such as of Patent Literature 2.
More particularly, the fin and tube heat exchangers used in the air conditioners and automotive radiators have the life of at most ten and several years, and a required life of corrosion resistance is such a relatively short time as just mentioned. In this connection, however, plate heat exchangers using sea water as cooling water, such as a vaporizer for natural liquefied gas, are industrially employed in plants and are of a large-scale equipment and thus expensive. Accordingly, it is required that the life and durability life of the heat exchanger be a semipermanent life of not less than several tens of years.
The corrosion resistance of such long life-oriented plate heat exchangers using sea water as cooling water is predominant of adhesion of a coating film to an aluminum alloy material rather than a corrosion resistance of the coating film itself. This is because if the coating film comes off in use, it is meaningless no matter how excellent a corrosion resistance of the film is. In other words, it would not be an overstatement to say that the corrosion resistance, such as a sea water corrosion resistance, of the long life-oriented plate heat exchanger using sea water as cooling water is an adhesion of a coating film to an aluminum alloy material. Such an adhesion of a coating film is impeded owing to the swelling of a coating film with a corrosive fluid such as seat water or the like.
In this regard, the anticorrosive method of directly providing a trifluoride resin anticorrosive coating film on a surface of aluminum alloy material as in the Patent Literature 1 and the Non-patent Literature 3 has a practical problem in that an adhesion to aluminum alloy materials is poor, and a corrosion resistance under use of sea water cannot be improved in a substantial way.